The present invention relates to electronic devices such as a RF (radio frequency) modulator, and particularly relates to a casing for receiving circuits constituting the devices therein to electrically shield the devices.
FIGS. 1(A) and 1(B) show cross sections of a typical example of a conventional casing for an electronic device.
In FIGS. 1(A) and 1(B), reference numeral 1 designates a frame formed by metal plates constituting side panels and a bottom panel of a casing. A circuit board 2 is fixedly mounted at a portion towards a rear opening in the frame 1. On the right half portion A of the circuit board 2 in FIG. 1(A), an input selection circuit is mounted, while on the left half portion B thereof a circuit constituting a RF modulator is mounted. The space within the frame 1 is divided into two sections by a shield plate 3 so as to electrically shield the input selection circuit from the RF modulator. The shield plate 3 is formed by bending a portion of the frame 1. As shown in FIG. 1(B), a tip portion of the shield plate 3 has two portions extending to the opening of the frame 1 past the circuit board. A tip portion of each of the two portions of the shield plate 3 is bent rectangularly thereby forming a contact portion 3a. A cover 4 formed by a metal plate is placed on the opening of the frame 1. The contact portions 3a are in contact with an inner surface of the cover 4 thereby grounding the shield plate 3 to the cover 4. Reference numerals 5a and 5b designate input and output connector plugs, respectively.
In the above-described conventional casing for electronic devices, the electric characteristics of the device vary greatly depending on whether the shield 3 is stably in contact with the inner surface of the cover 4 or not. However, in the conventional casing, since the main portion of the cover 4 is formed to be flat, the flat main portion is likely to be curved when forming the cover. As shown in FIGS. 2(A) and 2(B), if the main portion is curved outwardly, the cover 4 is away from the contact portions 3a of the shield plate 3 thereby electrically disconnecting the cover 4 from the shield plate 3. In this state, the shield characteristic of the casing is deteriorated thereby causing undesirable radiation of electric signals and the generation of cover noises.